1. Field of the Invention
The present invention relates generally to electronically steerable antenna systems, and is more particularly related to a phased array utilizing n-bit phase quantization.
2. Discussion of the Background
Communication and radar systems provide sophisticated applications that require accurately directing high-gain beams toward distant receivers/transmitters or targets. Phased array antennas are particularly suited to electronically steering directive beams, as the individual array elements can be controlled independently to exhibit a particular amplitude and phase. Advances in the capability to individually control these array elements without mechanical motion have lead to enhancements in scanning speed and improved ability to program the beams. In operation, a scanning beam changes direction incrementally in one or both of azimuth and elevation.
Phase shifters are widely used to accomplish electronic beam steering. The ability to control these phase shifters determines the speed and accuracy of switching beams; with rapid switching of beams, for example, a radar system is able to perform multiple functions, allowing the radar to track numerous targets. Further, steerable antennas are used, for example, in subscriber terminals and in earth stations that have to track a communication satellite. For electronic steering, traditional phased array antennas are expensive and are based on conventional phase shifters.
The phase shifting devices in modern phase array antenna systems are typically digital phase shifters. Unfortunately, digital phase shifters produce a phase quantization error, which increases the pointing error of the antenna beam and antenna pattern sidelobe levels. That is, the use of digital phase shifters for beam steering or other purposes introduces quantization errors, which degrades the antenna performance. Recent applications of phased array radars require higher angular measurement for antenna beam steering accuracy, thereby requiring the phase quantization errors of digital phase shifters to be reduced significantly.
FIG. 11 shows one conventional approach to design of a phased array antenna utilizing a 2-bit phase shifter. In this approach, a circular patch 1100 includes eight taps that are situated around the periphery of the circular patch 1100 at positions 1-8. Each of the taps has a corresponding switch; that is, taps 1-8 are connected to switches 1101-1108, respectively. Four phases (0xc2x0, 90xc2x0, 180xc2x0, and 270xc2x0) are generated by switching on the appropriate set of switches 1101-1108 associated with a pair of taps, which are directly opposite each other. Table 1, below, lists the pairing of taps and the corresponding phase.
As indicated in the above table, taps 1 and 5 are shorted using switches 1101 and 1105 to signify a 0xc2x0 (360xc2x0) phase, while the remaining taps 2-4, and 6-8 are open. Switches 1102 and 1106, which are connected to taps 2 and 6, respectively, are closed to yield 90xc2x0. Taps 3 and 7 are shorted to obtain 180xc2x0; i.e., the corresponding switches 1103 and 1107 are closed. Further, closing switches 1104 and 1108, associated with taps 4 and 8, results in a 270xc2x0 phase.
Under this approach, the circular patch 1100 is shorted to ground at the opposite ends of only one axis to support circularly polarized (CP) waves. The shorted points are effectively moved around the periphery to achieve phase shifts for CP waves. This type of patch 1100 requires different EM (electromagnetic) modes in the two orthogonal axes; and hence, the axial ratio is negatively affected. Recognizing that signals emitted from the circular patch 1100 possess orthogonal field components, it is observed that the above scheme of shorting opposite taps imposes different boundary conditions on the signals. For instance, to obtain a 0xc2x0 phase, taps 1 and 5 are shorted; however, the orthogonal taps 3 and 7 are open. Accordingly, the different boundary conditions exist for the orthogonal EM modes associated with the 0xc2x0 phase signal. Therefore, axial ratio degradation poses a problem, resulting in poor system performance.
Turning back to the issue of quantization error arising from the use of digital phase shifters, traditional antenna systems employ relatively high quantization levels to minimize such error. For example, 3 or 4-bit phase quantization has been used in phase shifter designs for beam steering to achieve acceptable performance from arrays. Coarser quantization, such as 2-bit phase quantization, has been viewed as lacking in performance. Additionally, conventional implementation of n-bit quantized phase shifters is prone to RF (radio frequency) losses. Nonetheless, higher bit phase quantization has been widely deployed. However, such a solution results in greater complexity and cost. Therefore, it is desirable from the perspective of reduced circuit complexity to use coarse quantization.
FIG. 12 shows the effect of 2-bit quantization in a conventional phase array antenna. As seen in the figure, strong quantization sidelobes appear. The presence of such prominent sidelobes impacts directivity negatively. Further, these sidelobes can potentially cause a violation of the emission specification of the antenna system.
Based on the foregoing, there is a clear need for improved approaches for providing electronically steerable antennas.
There is also a need to simplify the control circuitry associated with the array antenna.
There is also a need to enhance performance of the array antenna by reducing interference and signal degradation.
There is also a need to reduce the production costs of the array antenna.
Based on the need to increase antenna efficiency and minimize cost, an approach for electronically steering a beam utilizing simplified circuitry without reducing performance is highly desirable.
According to one aspect of the invention, a method is provided for performing electronic beam steering using a circular patch that has a plurality of taps coupled to a plurality of switches. The method includes controlling the plurality of switches to short an orthogonal pair of the plurality of taps. The taps are arranged on the circular patch to provide identical boundary conditions. The method also includes selectively performing at least one of receiving a signal and transmitting the signal. Under this approach, antenna circuitry is simplified, thereby reducing production cost.
According to another aspect of the invention, an antenna apparatus comprises a circular patch having a plurality of orthogonal tap pairs arranged on the patch. A plurality of switches correspond to the taps, in which each of the switches is coupled to ground. A controller is configured to control the switches to short one of the orthogonal tap pairs corresponding to a phase shift of a signal having orthogonal EM (electromagnetic) modes. The above arrangement advantageously provides enhanced performance of the antenna system.
According to another aspect of the invention, an antenna system for beam steering comprises a plurality of antenna elements being spaced according to a predetermined distance value. Each of the plurality of antenna elements includes a circular patch that has a plurality of orthogonal tap pairs arranged on the patch. A plurality of switches are correspondingly connected to the taps, in which each of the switches is coupled to ground. A plurality of phase perturbing sources are coupled to the taps to apply a fixed phase bias of a predetermined distribution. The predetermined distribution is at least one of a deterministic distribution and a random distribution. A controller is configured to control individually the plurality of antenna elements. The controller controls the switches of one of the plurality of antenna elements to short one of the orthogonal tap pairs corresponding to a phase shift of a signal having orthogonal EM (electromagnetic) modes, wherein the circular patch as controlled by the controller provides identical boundary conditions for the orthogonal EM modes. The above arrangement advantageously provides reduced signal degradation.
In yet another aspect of the invention, a computer-readable medium carrying one or more sequences of one or more instructions for performing electronic beam steering using a circular patch having a plurality of taps coupled to a plurality of switches is disclosed. The one or more sequences of one or more instructions include instructions which, when executed by one or more processors, cause the one or more processors to perform the step of controlling the plurality of switches to short an orthogonal pair of the plurality of taps. The taps are arranged on the circular patch to provide identical boundary conditions. Another step includes selectively performing at least one of receiving a signal and transmitting the signal. This approach advantageously reduces the number of switch components in an antenna system.